Glass fibers having improved strength

ABSTRACT

A glass fiber and a method of manufacturing a glass fiber for reinforcing a transparent composite matrix are disclosed. The glass fiber includes a first glass material having a first set of mechanical properties including a first modulus and a first coefficient of thermal expansion (CTE) and a second glass material having a second set of mechanical properties including a second modulus and a second CTE. The second glass material forms a substantially uniform coating on the first glass material. The second CTE is less than the first CTE. The glass fiber is formed by reducing the cross-section of a glass fiber preform of the first glass material coated with the second glass material by hot working. Because of the selected difference in the CTE&#39;s, the first glass material imparts a compressive force upon the second glass material, which improves the strength of the glass fiber.

FIELD

The present disclosure is directed to transparent reinforcing materialsand reinforced composite materials, more particularly to transparentglass fibers used in composite materials and to a process for producingsuch fibers.

BACKGROUND

Transparent composite materials are known for use in vehicle and otherapplications requiring light transmission or visual transparency. Suchtransparent composite materials include windows or other transparentmaterials useful for light transmission there through, particularly inrugged environments and in locations requiring ballistic resistance.Such reinforcement further provides the window or transparent devicewith improved strength.

Transparent composite materials typically include a reinforcing fiber ina polymeric matrix. In order to render the composite materialtransparent, both the matrix material and the reinforcing fiber arefabricated from a transparent material. The materials are typicallyselected to include the same optical properties, thus minimizingdistortion.

The geometry of reinforcing fibers also affects the distortion impartedto the light passing through the transparent device. For example, roundfibers (i.e., fibers having a circular cross-section) provide prismaticor other optical light refractive effects that provide overalldistortion of the light passing through the transparent device.

These transparent composite materials are also required to withstandhigh impacts and structural loads, and thus are required to have highstrength properties. The strength of these composite materials isdependent upon both the strength of the matrix material and thereinforcing fibers.

The strength of the reinforcing fibers is determined by the fibermaterial, geometry and characteristics imparted to the fibers duringmanufacture, such as surface cracks, imperfections, and otherinconsistencies.

What is needed is a fiber reinforcing material having improved surfacecharacteristics over the prior art.

SUMMARY

A first aspect of the disclosure includes a glass fiber for reinforcinga transparent composite matrix. The fiber includes a first glassmaterial having a first set of optical characteristics including, butnot limited to, a first refractive index (RI), a first modulus ofelasticity (modulus), and a first coefficient of thermal expansion(CTE), and a second glass material having a second set of opticalproperties including, but not limited to, a second RI, a second modulus,and second CTE. The second glass material forms a substantially uniformcoating on the first glass material. The second CTE is less than thefirst CTE.

Another aspect of the disclosure includes a method for fabricating aglass fiber for reinforcing a transparent composite matrix includingproviding a first glass fiber preform having a first RI, a firstmodulus, and a first CTE, coating a second glass material having asecond RI, second modulus, and a second CTE, substantially around across-section of the first glass material to form an initial coatedglass fiber preform having an initial cross-section, and hot working theinitial coated glass fiber preform to reduce the initial cross-sectionto a final cross-section of the glass fiber. The second CTE is selectedto be less than the first CTE.

Other features and advantages of the present disclosure will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section view of a glass fiber accordingto an exemplary embodiment of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

“Transparent”, “transparency” and grammatical variations thereof includean ability of a material to permit passage of at least a portion oflight directed at the material, the term “light” including anywavelength range of interest, and more particularly to the visible, nearvisible and near infra-red light ranges from about 380 nm to about 1000nm.

Referring to FIG. 1, a cross-section view of an exemplary glass fiber100 according to the disclosure is shown. As can be seen in FIG. 1,glass fiber 100 includes a first glass material 110 and a second glassmaterial 120. The second glass material 120 forms a substantiallyuniform coating around the first glass material 110. The glass fiber 100has a generally rectangular cross-section geometry having a totalthickness T, a first material thickness T₁, a second glass materialcoating thickness T₂, and a width W. The first glass material 110 andthe second glass material 120 may both be transparent glass. In oneembodiment, the first glass material 110 and the second glass material120 may both be transparent optical glass.

In alternative embodiments, the glass fiber 100 may have a differentcross-section geometry, for example, but not limited to generallysquare, generally oval, generally round and other similar geometries.

In one embodiment, the glass fiber 100 has a total thickness T ofbetween about 1 um to about 500 um with an aspect ratio of width W tototal thickness T of between about 5 and about 500. In anotherembodiment, the glass fiber 100 has a total thickness T of between about5 um and about 50 um with an aspect ratio of width W to total thicknessT of between about 10 and about 50.

In one embodiment, the second glass material coating thickness T₂ isbetween about 0.1% and about 100% the first glass material thickness T₁.In another embodiment, the second glass material coating thickness T₂may be between about 50 nm and about 5 um. In yet another embodiment,the second glass material coating thickness T₂ may be between about 50nm and about 1 um.

In another embodiment, the glass fiber 100 has a width W of betweenabout 5 um and 5000 um and an aspect ratio of width W to total thicknessT of between about 5 and about 500. In yet another embodiment, the glassfiber 100 has a width of between about 100 um to about 500 um and anaspect ratio of width W to total thickness T of between about 10 andabout 30.

The first glass material 110 is selected to have a first set ofproperties, including but not limited to a first RI, a first Abbenumber, a first transmission, a first modulus and a first CTE. Thesecond glass material 120 is selected to have a second set ofproperties, including, but not limited to a second RI, a second Abbenumber, a second transmission, a second modulus and a second CTE. Thesecond CTE is selected to be less than the first CTE.

The second glass material 120 must be chemically compatible with thefirst glass material 110. Furthermore, the second glass material 120must not contain elements that will negatively affect the desirableproperties of the first glass material 110 during the method of formingthe glass fiber 100 or in the glass fiber 100.

Additionally, the second glass material 120 must be thermally compatiblewith the first glass material 110 to facilitate forming the fiber 100.For example, the second glass material 120 must have approximately thesame viscosity versus temperature profile at hot working temperatures asthe first glass material.

In one embodiment, the second CTE is between about 0% to about 100% lessthan the first CTE. In yet another embodiment, the second CTE is betweenabout 70% to about 90% less than the first CTE.

In one embodiment, the first RI is approximately equal to the RI of apolymeric material in which the glass fiber 100 is used to form acomposite structure. The composite structure may be a window. In anotherembodiment, the first RI is substantially different form the second RI.In yet another embodiment, the first RI is approximately equal to thesecond RI.

In another embodiment, the second modulus is about equal to or less thanthe first modulus. In another embodiment, the second modulus is betweenabout equal and about 60% less than the first modulus.

In one embodiment, the second glass material has approximately the sameoptical performance as the first glass material. The second glassmaterial may also have approximately the same viscosity versustemperature profile at hot working temperatures as the first glassmaterial to facilitate forming the glass fiber.

The glass fiber 100 may be formed by the following exemplary embodiment.First, a first glass material preform of the first glass material isformed having a desired cross-section geometry and aspect ratio. Thecross-section of first material glass preform may be generallyrectangular, generally square, generally circular, or other similarshape. The first glass material preform may be formed by drawing,spinning, machining or other similar process.

The first glass material preform is then coated with a substantiallyuniform coating of the second glass material. The first glass materialmay be chosen for a desired optical performance. The second glassmaterial is chosen for it's lower CTE compared to the CTE of the firstglass material.

The second glass material may be coated onto the first glass materialpreform by slumping, chemical vapor deposition, plasma vapor deposition,sol-gel processing, slurry coating, or other similar process.Alternatively, a coating of the second glass material may be formed onthe first glass material preform by modifying the surface composition ofthe first material preform by methods such as, but not limited to,reactive chemical diffusion. By forming the second glass material bymodifying the surface of the first glass material preform composition,the second material has a compositional gradient varying from that ofthe surface of the coated first glass material preform to the firstglass material preform composition. The material properties of themodified surface would also have a gradient from the properties of thesecond glass material at the surface to the properties of the firstglass material preform material at some predetermined distance from thesurface. The composition and property gradient may be abrupt or gradualin nature.

In one embodiment, the first glass material preform has a generallyrectangular cross-section having a thickness of between about 0.5 mm andabout 12.7 cm and an aspect ratio of width to thickness of between about5 and about 500.

In another embodiment, the second glass material coating thickness onthe first glass material preform is between about 1 um and 25.4 mm.

In another embodiment, the second glass material coating thickness isbetween about 0.1% and about 100% the thickness of the first glassmaterial preform.

The coated glass fiber preform is then drawn under heat and pressure bymethods well known in the art to form a glass fiber having a rectangularcross-section geometry having a total thickness, a first materialthickness, a second glass material coating thickness, and a width asdiscussed above. The glass fiber may be formed in a continuous,semi-continuous, or step process. In one embodiment, the coated glassfiber is provided as a stock material that is later drawn to form aglass fiber.

The glass fiber may be used with an epoxy resin or other polymer to forma composite structure, such as a window, by methods appreciated by oneof ordinary skill in the art.

In one embodiment, the formed glass fiber has a second glass materialcoating thickness between about 0.1% and about 100% the first glassmaterial thickness. In another embodiment, the second glass materialcoating thickness may be between about 50 nm and about 5 um. In yetanother embodiment, the second glass material coating thickness may bebetween about 50 nm and about 1 um.

In another embodiment, the formed glass fiber has a total thickness ofbetween about 1 um to about 500 um with an aspect ratio of width tototal thickness of between about 5 and about 500. In another embodiment,the formed glass fiber has a total thickness of between about 5 um andabout 50 um with an aspect ratio of width to total thickness of betweenabout 10 and about 50.

In another embodiment, the formed glass fiber has a width of betweenabout 5 um and 5000 um and an aspect ratio of width to total thicknessof between about 5 and about 500. In yet another embodiment, the glassfiber 100 has a width of between about 100 um to about 500 um and anaspect ratio of width to total thickness of between about 10 and about30.

To form the glass fiber, the first glass material is selected to have afirst set of optical properties, including but not limited to a firstRI, a first Abbe number, a first transmission, a first modulus and afirst CTE, and the second glass material is selected to have a secondset of optical properties, including, but not limited to a second RI, asecond Abbe number, a second transmission, a second modulus and a secondCTE. The second CTE is selected to be less than the first CTE.

The second glass material 120 must be chemically compatible with thefirst glass material 110. Furthermore, the second glass material 120must not contain elements that will negatively affect the desirableproperties of the first glass material 110 during the method of formingthe glass fiber 100 or in the glass fiber 100.

Additionally, the second glass material 120 must be thermally compatiblewith the first glass material 110 to facilitate forming the fiber 100.For example, the second glass material 120 must have approximately thesame viscosity versus temperature profile at hot working temperatures asthe first glass material.

In one embodiment, the formed glass fiber includes a second glassmaterial having a second CTE between about 0% to about 100% less thanthe first CTE. In yet another embodiment, the second CTE is betweenabout 70% to about 90% of the first CTE.

In another embodiment, the formed glass fiber includes a first glassmaterial having a first modulus and a second glass material having asecond modulus, the second modulus is about equal to or less than thefirst modulus. In another embodiment, the second modulus is betweenabout equal and about 60% less than the first modulus.

In one embodiment, the formed glass fiber has a first RI approximatelyequal to the RI of a polymeric material in which the glass fiber 100 isused to form a composite structure. The composite structure may be awindow. In another embodiment, the first RI is substantially differentfrom the second RI. In yet another embodiment, the first RI isapproximately equal to the second RI.

In one embodiment, the formed glass fiber has a first glass materialhaving approximately the same optical performance as the second glassmaterial. In another embodiment, the formed glass fiber has a firstglass material having approximately the same viscosity versustemperature profile at hot working temperatures as the second glassmaterial.

In one example, a glass fiber is formed by selecting an optical glassN-SSK8 produced by SCHOTT North America, Inc., of Elmsford, N.Y., as afirst glass material. This optical glass has a set of opticalproperties, including but not limited to an RI, a Abbe number, atransmission, a modulus and a CTE of 7.21 e⁻⁶/C. A second glassmaterial, having a lower CTE is then used to coat the first glassmaterial and form a glass fiber. The second glass material is selectedto be chemically compatible with the first glass material. Additionally,the second glass material is selected to be thermally compatible withthe first glass material to facilitate forming the glass fiber. Forexample, the second glass material must have approximately the sameviscosity versus temperature profile at hot working temperatures as thefirst glass material.

During the hot forming of the glass fiber, the second glass material,because of the lower CTE, will impart a tensile force upon the firstglass material during and after cooling. This results in the secondglass material having compressive stresses after formation. Because thestrength of the glass fiber is directly related to the presence andgrowth of inconsistencies on the outer surface, the strength of theglass fiber will be increased by mismatching the CTE's of the first andsecond glass materials, as discussed above. This is because thecompressive stress formed in the second glass material because of theshrinkage will cause the second glass material to resist crack formationand crack propagation.

While the disclosure has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that thedisclosure not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure, but that thedisclosure will include all embodiments falling within the scope of theappended claims.

1. A method for fabricating a transparent composite matrix, comprising:providing a first glass fiber preform comprising a first glass materialhaving a first refractive index, a first modulus and a first coefficientof thermal expansion; and coating the first glass fiber preform with asecond glass material having a second refractive index, a second modulusand a second coefficient of thermal expansion to form a glass fiberpreform having an initial cross-section with a preform total thickness;and hot working the glass fiber preform to reduce the initialcross-section to a final cross-section having total thickness, athickness of the first glass material, and a substantially uniformcoating thickness of the second glass material; and forming a pluralityof the glass fiber into a composite window; wherein the secondcoefficient of thermal expansion is less than the first coefficient ofthermal expansion.
 2. The method of claim 1, wherein the secondcoefficient of thermal expansion is between about 0% and about 100% lessthan the first coefficient of thermal expansion.
 3. The method of claim1, wherein the second coefficient of thermal expansion is between about70% and about 90% of the first coefficient of thermal expansion.
 4. Themethod of claim 1, wherein the second modulus is between equal to andabout 60% less than the first modulus.
 5. The method of claim 1, whereinthe glass fiber has a cross-section geometry selected from the groupcomprising a substantially rectangular geometry, a substantiallycircular geometry, a substantially oval geometry, and a substantiallysquare geometry.
 6. The method of claim 5, wherein the cross-sectiongeometry is a substantially rectangular geometry.
 7. The method of claim6, wherein the glass fiber has a total thickness of between about 1 umto about 500 um and an aspect ratio of width to total thickness ofbetween about 5 and about
 500. 8. The method of claim 6, wherein theglass fiber has a total thickness of between about 5 um and about 50 umand an aspect ratio of width to total thickness of between about 10 andabout
 50. 9. The method of claim 1, wherein the hot working is selectedfrom the group comprising extrusion, co-extrusion, hot drawing,spinning, and co-spinning.